
Here \(dN_\gamma /dE|_{\rm endpoint}\)  is the endpoint contributions
computed in Refs. [1]}, [2]}. As
presented in the references, they are cascading photons from
monochromatic photons. Although the gamma-ray spectrum is determined
for fixed \(m_{\rm dm}\) , we take \(\sigma v_{\rm line}\)
\(\sigma v_{\rm line}=\sigma v_{\gamma \gamma }+\frac{1}{2}\sigma v_{\gamma Z}\,,\) 
